Patent Document 1 discloses an asymmetric hollow-fiber gas separation membrane made of polyimide that is composed of 4,4′-(hexafluoroisoproylidene) diphthalic acid and biphenyl tetracarboxylic acid as a tetracarboxylic acid component, and diaminodiphenylene sulfones (the same as diaminodibenzothiophene=5,5-dioxides described later) and others as an main ingredient of a diamine component. This asymmetric hollow-fiber gas separation membrane, as is known from the Examples described in the document, has a hydrogen gas permeation rate (P′H2) of 71 to 88×10−5 cm3/cm2·sec·cmHg and a ratio of hydrogen gas permeation rate to nitrogen gas permeation rate (P′H2/P′N2) of 36 to 41. Although the membrane has such high gas permeation performance, there is still room for further improvement on its gas permeation performance. The document also discloses that an aromatic diamine compound having plural benzene rings is preferably used in combination with a diamine such as diaminodiphenylene sulfones, and that 3,5-diaminobenzoic acid or the like may be used in an amount of 10 mol % or less. However, there is not any description in the document about the use of 3,5-diaminobenzoic acid in an amount of more than 10 mol %.
Reference Patent Document 2 discloses a gas separation membrane made of a copolymer polyimide that is composed of 4,4′-(hexafluoroisoproylidene)-bis(phthalic anhydride) as the tetracarboxylic acid component, and 3,7-diamino-2,8-dimethyldiphenyl sulfone (the same as 3,7-diamino-2,8-dimethyldibenzothiophene=5,5-dioxide described later) and a hydrophilic diamine as the diamine component. The document describes that the gas separation membrane may use 4,4′-(hexafluoroisoproylidene)-bis(phthalic anhydride) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride in combination as the tetracarboxylic acid component, and further describes that 1,3-diamino-5-benzoic acid (the same as 3,5-diaminobenzoic acid described later) may be used as the hydrophilic diamine. However, in the Examples described in the document, only the gas permeabilities of carbon dioxide and methane are described for the uniform membrane (film) obtained by casting polyimide. Further, there is described no specific example of using 3,5-diaminobenzoic acid as a hydrophilic diamine. That is, there were no disclosures and implications about the polyimide that is composed of 4,4′-(hexafluoroisoproylidene)-diphthalic acid and biphenyltetracarboxylic acid as the tetracarboxylic acid component, and 3,7-diamino-2,8-dimethyldiphenyl sulfone and 3,5-diaminobenzoic acid as the diamine component, whether or not the polyimide is suitable for the production of an asymmetric hollow-fiber membrane, whether or not the film has an improved gas permeation performance for the separation of hydrogen gas, oxygen gas, nitrogen gas, and others, and whether or not the asymmetric hollow-fiber membrane has a practical mechanical strength required for hollow-fiber membranes in general.
Patent Document 1: Japanese Patent Laid-Open Publication No. H03-267130, and
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-516131.